"An FAA inspector will have to accept the work before the airplane technically is in compliance with the (directive)," said FAA spokesman Les Dorr.

Nearly 50 already-delivered Dreamliners have been grounded for more than three months, after two incidents on jets operated by Japanese airlines called the battery systems into question. The Dreamliner's use of lightweight composite materials to greatly improve fuel economy has made it a big seller in Asia and the Middle East, where long-haul flights account for much of an airline's business.

United Airlines, which has six Boeing 787 aircraft, is the only U.S. airline to take delivery of the Dreamliner so far. It will cost the airline about $2.8 million to implement the fix, according to the FAA's Federal Register filing.

Calling the FAA plan "a good step forward," United spokeswoman Christen David said the company is developing "a return-to-service plan, and we look forward to getting our 787s back in the air."

The company plans to begin domestic flights using the 787 in May and possibly launch the Denver-Narita (Japan) route on June 10.

Boeing is sending special teams to assist in the repairs, according to Boeing spokesman Marc Birtel.

"The FAA's publication of the Airworthiness Directive is an important step forward in returning the 787 fleet to flight status," Birtel said in an e-mail.

The directive requires that 787 operators install "main battery and auxiliary power unit battery enclosures (APU) and environmental control system (ECS) ducts; and replacing the main battery, the APU battery and their respective battery chargers."

"Once modifications are completed on individual airplanes, our customers will determine, with their regulatory authorities, when to return their 787s to service," said Birtel.

Although the FAA's fix only applies to U.S. carriers, it's likely other foreign civil aviation authorities will consider the U.S. agency's plan, said the FAA's Dorr.

Boeing has already started installing the modified battery system on 10 aircraft in service and nine aircraft in production, said Boeing CEO Jim McNerney during Boeing's earnings call Wednesday.

"We expect to complete the bulk of fleet retrofits by mid-May," said McNerney, according to a transcript of the call.

McNerney said Boeing expects to resume Dreamliner deliveries in early May.